For planetary gear transmission units upon which extremely high demands are made and which are subjected to very high loads, such as planetary gear transmission units for wind turbines for example, gears with helical teeth are usually used, since gears with helical teeth possess better characteristics both for achieving a required nominal capacity and a reduction of sound and vibration.
Some kinds of such planetary gear transmission units with helical teeth are already known. However, they still have major problems and could still be optimised considerably.
When designing a planetary gear transmission unit, a selection has to be made regarding the helix angle of the gear teeth and the dimensions to be used for the ring gear, sun gear and the planet gears, in order to be capable of taking the required load and to achieve the correct gear ratio.
In order to be capable of withstanding higher loads, it may be possible to extend the dimensions of the entire gear transmission unit, which should, however, be limited as far as possible, of course, for economic and logistic reasons.
Only a proper combination of all possible factors may result in the creation of a gear transmission unit that can take higher loads while having relatively small dimensions, at least in comparison with existing planetary gear transmission units.
For example, in case of bearings for supporting planet gears in the planetary gear transmission unit, there are a number of restrictions when selecting such planet bearings with larger radial dimensions, as the gear rim of these planet gears must have a certain thickness to avoid negative interactions between the teeth of the planet gears and the outer bearing ring of the planet bearing, or simply to withstand the loads or to ensure a certain minimum lifespan for the bearing.
Along the axial direction, the required gear capacity imposes a minimum value on the gear width, and sufficient gear width is also necessary in order to be capable of taking the torques on the planet gears by means of a bearing, or to achieve proper axial and radial bearing support for the planet gears. The helix angle of the gear teeth affects the planet bearing, since gears with helical teeth are inclined to axially move away from each other. The larger the helix angle of the teeth, the larger the axial forces between the teeth will be. A planetary gear transmission unit has this tendency for the gears to axially move apart, both between the ring gear and the planet gears, and between the planet gears and the sun gear.
The axial force to which a planet gear is subjected in relation to the ring gear is opposite to the axial force exerted by the sun gear on the planet gear. These axial forces therefore cancel each other out, as a result of which there is no net axial force seen at the planet shafts and the planet bearing, so that this does not affect the planet bearing.
A known solution for making the planetary gear transmission unit to withstand high loads is to use flexpin shafts as planet shafts. Such flexpin shafts are known from, for example, GB 1,101,131.
FIG. 1 illustrates the principle of a flexpin shaft. FIG. 1 shows a planet gear 1 which is mounted on a flexpin shaft 2 by means of bearing 3. Exerting a force F on the planet gear 1 causes a moment A on one side of the flexpin shaft 2 and a moment B on the other side of the flexpin shaft 2, moments A and B having opposite directions. This causes angular deformation of the flexpin shaft 2 as illustrated at the right side of FIG. 1. This angular deformation on both sides of the flexpin shaft 2 should be equal.
However, such a flexpin shaft is not suitable to be used in combination with gears having helical teeth. This is because in case of helical teeth, the opposite axial forces (see arrows with reference number 6 in FIG. 2) which were described above, are exerted at the ring gear and sun gear respectively. Therefore, each planet gear is subjected to tilted moments which have to be handled by the planet bearing. Hence, when using helical teeth in a planetary gear transmission unit, a moment is created by the axial components of the normal tooth forces in the ring gear and sun gear meshes respectively. This causes the planet gears 1 to skew. With a flexpin design the planet shaft assembly is less stiff than in conventional designs and will thus cause more planet skewing. This may be solved by making the flexpin shaft 2 anisotropic as far as its stiffness goes. This is illustrated in FIG. 3, where the flexpin shaft 2 is designed to have a part 4 with lower stiffness than the part 5. In that way, flexibility in the tangential direction is still allowed while the flexpin shaft is as stiff as possible in a plane normal to the tangential direction. In this way, it could become possible to use the flexpin in combination with helical teeth.
However, the above described solution is complex and therefore increases the cost and manufacturing time of planetary gear transmission units comprising such flexpin design.
Therefore, there is a need for a design of a planetary gear transmission unit in which the planet gears have helical teeth and which are able to withstand high loads.